The present invention relates to a method and composition for depositing silicon dioxide layers on substrates by chemical vapor deposition and is particularly directed to a method and composition for low temperature chemical vapor deposition of alternating layers of silica and titania.
It is known in the art that silicon dioxide layers can be formed by pyrolytic deposition of organosilicon reagents such as Silicon tetraethoxide, by chemical vapor deposition techniques. Although these alkoxy silanes provide acceptable silica layers, pyrolysis temperatures which approximate 600.degree. C. and above are required. Such high temperature requirements pose a problem where deposition of a secondary oxide layer at pyrolysis temperatures below 600.degree. C. is desired. There is a need in the art for a system which does not require thermal cycling of the deposition apparatus to provide alternating layers of silica and other oxides.
Alternating layers of silica and metal oxide are useful in many applications including the fabrication of optical filters capable of transmitting high-frequency radiation while reflecting low-frequency radiation. These filters comprise stacks of films with alternate indices of optical refraction and are described more particularly by Rancourt in U.S. Pat. No. 4,229,066. Silicon dioxide layers are used as the layer having a low index of refraction while some other metal oxides provide layers having a high index of refraction. So called high-pass optical filters can be used to increase the lumens per watt of an incandescent light bulb by freely transmitting visible light through the glass envelope while reflecting infrared radiation back onto the incandescent filament, thereby producing a higher filament temperature for a given input level of current.
While composition and method are especially useful in providing silicon dioxide layers for high-pass optical filters, the present invention contemplates the formation of silica layers, for other purposes, for example films used to make conformal silica layers for electronic chip passivation or dielectric insulation at transistor gates or the formation of silica layers on any suitable surface for electronic or optical utilization.